Detalles de publicación
PP 011048
The Outer Disks of Early-type Galaxies, II. surface Brightness Profiles of Unbarred Galaxies, and Trends with Hubble Type
1. UNAM, Ensenada Mexico, 2. IAC, 3. Max Planck Institut fúr Extraterrestresche Physic, Garching. (4) Universität-Sternwarte, München. 5. Guest Investigator of the UK Astronomy Data Centre. 6. Present Address, Dept. Phys. Astronomy, UCL (London), 7. Facultad de Física, ULL. 8. CSIC.
We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample
of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred
galaxies. Following very careful sky subtraction, the profiles can typically be determined down to
brightness levels well below 27 mag arcsec−2 and in the best cases below 28 mag arcsec−2. We
classified the profiles according to the scheme used previously for the barred sample: Type I profiles
are single unbroken exponential radial declines in brightness; Type II profiles (“truncations”) have an
inner shallow slope (usually exponential) which changes at a well defined break radius to a steeper
exponential; and Type III profiles (“antitruncations”) have an inner exponential that is steeper, giving
way to a shallower outer (usually exponential) decline.
By combining these profiles with previous studies, we can make the first clear statements about
the trends of outer-disk profile types along the Hubble sequence (including both barred and unbarred
galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type
disks, decreasing from one-third of all S0–Sa disks to barely 10% of the latest type spirals. Conversely,
Type II profiles (truncations) increase in frequency with Hubble type, from only 25% of S0 galaxies
to 80% of Sd–Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies
(Hubble types S0–Sm) are: 21%, 50%, and 38%, respectively; this includes galaxies ( 8% of the
total) with composite Type II+III profiles (counted twice).
Finally, we note the presence of bars in ten galaxies previously classified (optically) as “unbarred”.
This suggests that 20% of optically unbarred galaxies are actually barred; the bars in such cases
can be weak, obscured by dust, or so large as to be mistaken for the main disk of the galaxy.
of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred
galaxies. Following very careful sky subtraction, the profiles can typically be determined down to
brightness levels well below 27 mag arcsec−2 and in the best cases below 28 mag arcsec−2. We
classified the profiles according to the scheme used previously for the barred sample: Type I profiles
are single unbroken exponential radial declines in brightness; Type II profiles (“truncations”) have an
inner shallow slope (usually exponential) which changes at a well defined break radius to a steeper
exponential; and Type III profiles (“antitruncations”) have an inner exponential that is steeper, giving
way to a shallower outer (usually exponential) decline.
By combining these profiles with previous studies, we can make the first clear statements about
the trends of outer-disk profile types along the Hubble sequence (including both barred and unbarred
galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type
disks, decreasing from one-third of all S0–Sa disks to barely 10% of the latest type spirals. Conversely,
Type II profiles (truncations) increase in frequency with Hubble type, from only 25% of S0 galaxies
to 80% of Sd–Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies
(Hubble types S0–Sm) are: 21%, 50%, and 38%, respectively; this includes galaxies ( 8% of the
total) with composite Type II+III profiles (counted twice).
Finally, we note the presence of bars in ten galaxies previously classified (optically) as “unbarred”.
This suggests that 20% of optically unbarred galaxies are actually barred; the bars in such cases
can be weak, obscured by dust, or so large as to be mistaken for the main disk of the galaxy.